1. Field of the Invention
The present invention relates to a sawing wire for the simultaneous cutting and lapping of a multiplicity of wafers from a hard brittle workpiece. The present invention also relates to a method for cutting and lapping using a sawing wire.
2. The Prior Art
A wire saw which is suitable in particular for the production of semiconductor wafers, and the operating principle on which a wire saw is based, are described, for example, in U.S. Pat. No. 5,771,876.
Generally, wire saws are equipped with wire-guide rollers, over which an endless sawing wire is guided helically, so that a sawing harp or a cutting or wire web is formed. The workpiece is guided through the wire web by an advancing movement and is split into wafers when it passes through the web. The sawing wire is either moved in the same direction over the entire cutting run or is moved to and fro (oscillating wire movement). The cutting-lapping agent used is abrasive grains of different grain sizes and different hardnesses, which are suspended in a carrier liquid.
The quality of the cut, i.e. the quality of the wafer surfaces and the wafer geometries is dependent in the first instance on uniform distribution of the cutting-lapping agent along the width of the web. However, sawing wires according to the prior art generally do not carry the amount of cutting-lapping agent which is available on the wire web optimally into the cutting gap.
It is an object of the present invention to avoid the abovementioned drawbacks of the prior art, to improve the geometry of the wafers and surface parameters, such as the striations, roughness and rippling, and to shorten the process times.
The above object is achieved according to the invention by means of a method for the simultaneous cutting and lapping of a multiplicity of wafers from a hard brittle workpiece using a sawing wire which runs helically over wire-guide rollers and forms a wire web, and a cutting-lapping agent in a wire saw, wherein the sawing wire executes a rotary movement during the cutting and lapping.
Surprisingly, it has been found that the use of a sawing wire with a defined torsion leads to a rotation of the wire during the cutting and lapping. This rotary movement, which is similar to a boring movement, promotes the distribution of the cutting-lapping agent on the wire web and in particular the supply of cutting-lapping agent to the cutting gap. Preferably, the sawing wire rotates while it is being guided over the wire web. The sawing wire preferably rotates 1 to 10 times, particularly preferably 2 to 5 times, over a wire length of 1500 mm.
Accordingly, this object of the invention is also achieved by means of a sawing wire for the simultaneous cutting and lapping of a multiplicity of wafers from a hard brittle workpiece, wherein the sawing wire exhibits torsion.
The torsion of the sawing wire is generated by the wire being turned about its own axis, for example by oppositely directed rotation from the two ends. Preferably, the torsion of the sawing wire can be generated by rewinding using an unwinding arm. For example there can be a sawing wire according to the prior art being unwound from a wire reel by means of an unwinding device, known as a flyer, and being wound back onto a sawing wire reel vertically or horizontally. This unwinding and rewinding results in a defined torsion being imparted to the sawing wire. The sawing wire reel bearing the sawing wire according to the invention is then used as a so-called source or dispenser reel in a wire saw according to the prior art.
However, it is also preferable to use a wire saw reel having a sawing wire in accordance with the prior art as a source or dispenser reel in a wire saw and to turn the sawing wire by means of a twisting device. The twisting device is desirably positioned in front of or behind the wire web.
Over a sawing wire length of preferably 150 to 1500 mm, particularly preferably from 300 to 1000 mm, the torsion of the sawing wire amounts to 360xc2x0. Over a wire length of preferably 150 to 1500 mm, and particularly preferably of 300 to 1000 mm, a sawing wire is turned through 360xc2x0 by means of a twisting device. Desirably, the twisting device is positioned in front of or behind the wire web. The sawing wires used are coated or uncoated wires with a thickness of from 80 to 500 xcexcm and a tensile strength of  greater than 2000 N/mm2.
The rotation of the sawing wire according to the invention during the cutting and lapping, in particular of hard brittle workpieces, leads to an improved surface quality of the wafers. In particular this leads to a reduced roughness and rippling in the surface of the wafers. Furthermore, improved geometry characteristics (wedge effect) are achieved and shortened process times are achieved.